In a bowl, combine the sliced flank steak with 1 tablespoon of melted butter, minced garlic, fajita seasoning, salt, and pepper. Let marinate for at least 15 minutes to absorb the flavors.
In a large skillet or frying pan, melt 1 tablespoon of butter over medium heat. Add the sliced bell pepper and red onion, cooking for about 5-7 minutes until they're tender and slightly caramelized. Remove from the pan and set aside.
In the same skillet, add the marinated steak to the hot pan. Cook for about 5-6 minutes, stirring occasionally until it reaches your desired doneness (medium-rare is about 135°F).
Once the steak is cooked, return the sautéed vegetables to the skillet. Mix everything together for 1-2 minutes to heat through and blend the flavors.
Add the remaining 2 tablespoons of butter to the skillet, stirring until melted and well incorporated.
In a separate pan, warm the flour tortillas on low heat for about 30 seconds on each side until soft and pliable.
Spoon generous portions of the steak and vegetable mixture onto the warm tortillas. Garnish with fresh cilantro and serve with lime wedges on the side for squeezing over the top.